1. Field of the Invention
The present invention relates to aromatic polyimide-polyamide polymers containing the hexafluoroisopropylidine or the 1-phenyl-2,2,2-trifluoroethane linking group having improved physical and chemical properties, and to a method for preparing such polymers.
2. Description of Related Art
Polyamide-imide polymers and copolymers are known in the art. These materials may be prepared by the condensation polymerization of trifunctional acid monomers such as the 4-acid chloride of trimellitic anhydride and one or more aromatic diamines. Examples of such polymers are disclosed in U.S. Pat. Nos. 3,347,828, 3,494,890, 3,661,832 and 3,920,612. These polymers may be characterized by extremely good high temperature properties (Tg of about 275.degree. C. or higher), good high temperature stability, good tensile strength at high temperatures, good mechanical properties and good chemical resistance. These polyamide-imides are useful as wire enamel coatings, laminates, molded products, films, fibers; impregnating varnishes and in other applications where high thermal stability is required.
Polyimide-polyamide polymers may also be prepared by reacting an aromatic diacid chloride, an aromatic diamine and an aromatic dianhydride as disclosed for example in U.S. Pat. No. 3,179,635.
One of the problems associated with such polymers is that they exhibit generally poor flow properties which render them difficult to process, particularly to injection mold or to spin into fibers. These polymers are also relatively hydrophilic and tend to absorb moisture which can effect their clarity, heat stability, processability, mechanical and electrical properties. Many of these polymers are also insoluble in organic solvents which renders them less suitable in film casting and fiber spinning applications and for the production of composites.
Attempts have been made in the art to improve the flow properties of polyamide-imide polymers. For example, U.S. Pat. No. 4,448,925 discloses including from about 1 to about 10 percent phthalic anhydride into the polymerization recipe to provide polyamide-imide copolymers having improved flow properties. However, this technique for improving flow properties is made at the expense of the heat stability and chemical resistance of the polymer. Yet another method for improving the flow properties of such polymers is to form blends thereof with up to about 10% by weight of a polyamide such as nylon 6 or nylon 66, as disclosed in U.S. Pat. No. 4,575,924. Once again however, such an approach to solving the flow problem is made at the expense of the thermal stability and optical clarity of the resultant polymer blend.